Rotational molding



Nov. 7, 1967 s. SCHOTT ETAL ROTATIONAL MOLDING 2 Sheets-Sheet l FiledMay 26, 1966 Nov'. 7, 1967 Filed May 26, 1966 LTl/ 1 2 Sheets-Sheet 2STUART SCHOTT LEROY J. MEMERING INVEN'TOR.

United States Patent Oiice 3,350,745 Patented Nov. 7, 1967 3,350,745ROTATIONAL MOLDING Stuart Schott, Cincinnati, Ohio, and Leroy J.Memering,

Covington, Ky., assignors to National Distillers and ChemicalCorporation, New York, N.Y., a corporation of Virginia Filed May 26,1966, Ser. No. 553,177 4 Claims. (Cl. 18-26) This invention relates toimprovements in the rotational molding of powdered plastics, and moreparticularly relates to a rotational molding apparatus and process whichprovides for more rapid cooling, more uniform cooling and cooling underan atmosphere to which the plastic is inert at the elevated temperatureof the molding process.

Rotational molding processes are well known for making hollow, dividedor open end plastic objects of various sizes from a powdered polymer.Any powdered polymer can be used, in the dry state or as liquiddispersion, which is capable of forming a lm under the influence ofheat. These powders are produced by mechanical reduction of commercialsize pellets in suitable grinders such as a Pallman mill or byprecipitation from a solution. Particles with smooth spherical shape tocoarse irregular shape can be used, and any particle size, shape anddistribution can be used which gives the desired wall thickness anddistribution of the polymer on the walls of the mold. Generally, smallerparticle size materials (5*15 microns) are used for thin wall itemswhile larger particles (30G-2000 microns) are used for thick wall items.Normally, powdered thermoplastics of nominal20 (840 microns) to 50 (297microns) mesh particle size are used in rotational molding for wallthickness of 0.050 to 0.250.

Basically, the molding process consists of placing the plastic in finelydivided form inside of a mold. The mold is then heated to a temperatureabove the melting point of the plastic and, at the same time, it isrotated on two perpendicular axes. The powdered plastic inside the moldis heated by the heat transfer from mold surface and sticks to the innermold surface. Heating is continued for a sulficient length of time forcomplete melting of all the plastic particles and to permit bubbles tobe released from the molten plastic. The thickness of the plasticarticle is determined by the amount of plastic placed within a givenmold.

After heating is completed, the mold is cooled by contacting theexterior surface of the mold with air, water spray or by immersion inwater. The mold is normally vented to equalize the pressure during theheating and cooling cycles. After the cooling cycle is finished, themold is opened and the molded plastic article removed.

Short cycle times for heating and cooling are desired for economicreasons. In addition, most plastics are heatsensitive, so that minimumheating temperature and time are used consistent with complete fusion ofthe plastic particles and removal of bubbles from the finished article.Once the fusion and removal of bubbles are accomplished, cooling shouldbe carried out as quickly as possible to give a `molded article of highimpact strength. However, as mentioned above, cooling in the prior artis obtained by transferring heat from the outer surface of the hotplastic to the mold. Since this cooling occurs almost entirely from theouter surface of the molded article, strains are set up because of largevariations in temperature between the inner and outer surface of theplastic article with resultant uneven contraction of the plastic. Thisproblem is aggravated further since the plastic shape, as it cools,shrinks away from the mold due to the higher coeicient of expansion ofthe plastic compared to metal. This results in slower cooling after thisshrinkage has occurred.

A further disadvantage in the prior art molding processes is thatcooling of the plastic by heat conduction into the hollow space in itsinterior is minimal because there is no movement of the enclosed gas tothe exterior other than the breathing through the vent tube.

In accordance with the invention, a cooling lluid is circulated throughthe interior of the plastic object under a slight positive pressure andvented to the atmosphere following the heating cycle which fluid couldbe a stream of air, or preferably an inert gas, to provide for coolingthe interior surface of the plastic object while the exterior surface isbeing cooled by the mold in the usual prior art manner which, in turn,is being cooled by air circulation or water spray or water immersion.'In addition, the slight internal pressure aids in holding the object tomold surface.

By inert gas is meant `a gas which does not degrade the plastic byreaction with it at the highest temperature used in rotational molding.If desired, preliminary cooling of the interior can be carried out withan inert gas until the temperature of the plastic surface has lowered to-a sufficient extent to make it non-reactive with air, following whichair can be passed through the molded plastic in place of the inert gas.Thus, the invention provides all of the advantages of quick cooling,eg., Ireduced molding cycle and waste, therefore lower manufacturingcost, increased impact strength and absence of discoloration. At thesame time, there is obtained a product with a minimum of undesirablewarpage.

It is an object of the present invention to provide for rapid cooling ina rotational molding process without undesirable distortion or warpageof the plastic.

It is also an object of this invention to provide rapid cooling in arotational molding process to give a molded object of high impactstrength.

Another object of this invention is to provide -rapid cooling in arotational molding process in which the molded plastic is exposed tohigh temperatures for the minimum period of time.

Still another object of this invention is to provide for the rapidcooling of plastic in a rotational molding structure under an inertatmosphere to provide a plastic object which has not been discolored byreaction of the hot plastic with cooling fluid.

Another object of this invention is to maintain good conformity of themolded plastic article to the mold pattern.

These and other objects of this invention will become apparent from thefollowing description when taken in connection with the drawings, inwhich:

FIGURE l 'shows a rotational molding apparatus in cross-section, asmodified in accordance with the invention.

FIGURE 2 shows a cross-section through the vent cap that isconnectableto the mold lid of FIGURE 1.

FIGURE 3 shows a cross-section through the internal cooling cap that isshown connected to 'the lid of FIG- URE l for internal mold cooling.

Referring first to FIGURE 1, the apparatus of the invention is containedin a suitable insulated hot air oven (not shown) which is heated toBSO-950 F., or

-14. The other end of shaft 12 is suitably rigidly connected to rotatingframe 15.

Frame 15 carries a bearing 16 which rotatably mounts chain-gear sprocket17. A chain then connects rotating sprocket 17 to a fixed chain-gearsprocket 18 which is fixed to bearing 11. Chain-gear sprocket 17 thenhas a shaft 18a keyed thereto at one end. The other end of shaft 18acarries bevel gear 19 which is keyed to shaft 18a and meshes with bevelgear 20.

Bevel gear 20 is keyed to shaft 2l which is rotatably mounted onbearings 22a and 22b which are carried on support structure 23 which issuitably mounted from rotating frame 15, Shaft 21 is then fixed to plate24 which is, in turn, connected With drive pins to rotating cage 25.Cage 25 then has mounting pedestals 26 and 27 extending therefrom whichserve to mount mold 28 as by clamping or welding, or the like.

The mold 28 is made of sheet metal, but any material suitable for amold, eg., cast aluminum or electroformed copper, may be used. Mold 28has an interior shape which conforms to the exterior of the articlebeing formed. In the example used to illustrate the invention, the mold28 has a box shape, the interior of which is to be lined with the moldedarticle 29. The box shape mold herein described is intended toillustrate but not limit the mold which can have any desiredconfiguration.

Up to this point, the apparatus is of the well known type. In accordancewith the invention, the mold lid 30 is of a novel construction and hasthe vent cap 31 of FIGURE 2, or the internal cooling head 32 of FIGURE3, selectively connected thereto by the clamp 33 in FIG- URE l.

FIGURE 1 illustrates cooling head 32 in position in lid 30. The lid 30is lined with an insulation panel 33a and has a central opening 34therein. The central opening 34 is large enough to receive either vent31 of FIGURE 2 or cooling head 32 of FIGURE 3.

Clamp 33 is secured to lid 30 and includes an operating lever 35 whichoperates a lever 36 through a suitable overcenter toggle mechanism 37.Lever 36 has a pressure pad 38 adjustably connected thereto which clampsatop the rod 39 of vent 31 of FIGURE 2 or rod 40 of cooling head 32 ofFIGURE 3 to hold the vent 31 or cooling head 32 in position on lid 30.That is, to place vent 31 in position on lid 30, the vent body 41 ofFIGURE 2 is placed in opening 34 and the plate 42 seats atop plate 30 toprevent vent 31 from passing through opening 34. The hollow vent tube 43then communicates between the interior of mold 28 and the exterioratmosphere. Pressure pad 38 of clamp 33 is then locked on rod 39 torigidly hold vent 31 in position.

The cooling head is connected to lid 30 in a similar manner Where,however, the cooling head has a plate 50 with a plurality of openingstherein such as openings 51 and 52 which are evenly spaced around theperimeter to keep head 32 from passing through opening 34 in lid 30. Thecooling head 32 then includes a nozzle portion 53 having a plurality ofinternal channels such as channels 54 and 55 which lead to a commonchannel 56 which extends through plate 50.

The channel 56 is then connected to conduit 57 which passes through asupport bearing 58 connected to frame 15 and through a rotary union 59.Rotary union 59 then has an input conduit 60 which is connected to frame15 and communicates through the center of shaft 12 with rotary union 61.A suitable input conduit connected to rotary union 61 is then connectedto a cooling uid source 62 through valve 63.

External cooling jets of the usual type are also provided, as shownschematically in FIGURE 1, by nozzles 70 and 71 connected to coolingfluid source 72.

In operation, the mold and lid are coated with a suitable release agentsuch as Dow-Corning DC-20 silicone resin. The plastic load is placed inmold 28 and the vent 31 is clamped in lid 30 to permit gas ventingduring the molding operation. The assembly is then placed in a suitableoven, and drive motor 14 is turned on. Motor 14 causes sprocket 13 torotate which causes shaft 12 and frame 15 connected thereto to rotate.This cornponent of rotation causes rotation of cage 25 about an axiscolinear with shaft 12. Since sprocket 18 is stationary, the rotation`of sprocket 17, carried on frame 15,

about sprocket 18 will cause shaft 18a to rotate with respect to frame15, thereby rotating shaft 21. This, in turn, causes cage 25 to rotateabout an axis colinear with shaft 21.

Thereafter, the mold is rotated in the oven in this manner for asuflicient time to melt the plastic and eliminate air bubbles, with hotair and gas venting through the vent tube 43 of the vent 31. Therotating mechanism and mold are then removed from the oven and motor 14is deenergized to stop mold rotation.

Vent plug 31 is then removed and the cooling head 32 of FIGURE 3 isclamped into place by clamp 33, as shown in FIGURE 1. Motor 14 isreenergized to cause rotation of cage 25 and mold 28 so that the moltenpolymer 29 of FIGURE l irmly adheres to the mold Walls. Valve 63 is thenopened to permit a suitable c-oolant, such as gaseous nitrogen fromcooling fluid source 62 to be admitted into the mold interior throughthe cooling head 32 and to impinge on the mold interior at a slightpositive pressure. The hot cooling fluid is allowed to escape throughvents of plate 50, shown in FIGURE 3. At the same time, cooling fluidsuch as water is taken from source 72 and is applied to the moldexterior through suitable nozzles such as nozzles 70 and 71.

After the mold and product are cooled, the product can be removed forfurther treatment, e.g., removal of flash, decorating, packaging, etc.

The advantages of the invention will be illustrated in the followingexamples:

EXAMPLE 1 A linear polyethylene box with wall thickness of 0.12 inch wasmade by rotational molding using forced air cooling on the outside moldsurface. Impact strength of the box wall was 1100 ft. lbs/inch withslight Warpage.

EXAMPLE 2 A linear polyethylene box of the same size and wall thicknessas Example l was 'molded using the same heating cycle as Example 1, butcooled by spraying cold water on the mold. Impact strength of the boxwas in excess of 5000 ft. lbs/inch, but warpage was so bad thatobtaining a reasonably flat sample for testing was impossible.

EXAMPLE 3 The process of Example 1 was repeated, except that at the endof the heating cycle, the mold cover was removed and water sprayed inthe form of a line mist on the inside of the polyethylene box while theoutside of the mold was cooled by Water spray. There were slight markson the interior surface of the polyethylene box Where the Water dropletshad contacted molten polyethylene. Impact strength of the box wall wasover 5000 ft. lbs/inch and warpage was intermediate between Examples 1and 2. These examples show that cooling of the rotationally moldedplastic article from both sides gives the high impact advantage of rapidcooling with the low warpage advantage of slow cooling.

In order to test this invention in a practical manner, a gas fired, hotair rotational molding machine was modified so that inert gas could befed into the mold following the fusion of the powdered plastic insidethe mold without stopping the rotating machanism.

EXAMPLE 4 The process of Example 1 was repeated, except that at the endof the heating cycle compressed air at 15 p.s.i. was fed through acooling orifice through the inside of the mold and exhausted, while atthe same time Water was sprayed on the outside of the mold until thecooling period was over. Impact strength of the box wall was 4000 ft.lbs/inch, but Warpage was equivalent to Example 1, i.e., only slightwarpage. Thus, the benefits of fast cooling were obtained with respectto impact strength, and at the same time obtaining the benefit of slowcooling in the form of low warpage. In Examples 1 through 4, it wasnoted that a slight brownish discoloration of the plastic occurred,presumably due to oxidation by the contained air of the plastic at hightemperatures of molding.

EXAMPLE 5 The process `of Example l and Example 4 was repeated, with theexception that compressed nitrogen at 8 lbs. p.s.i. was fed to andthrough the inside of the mold and exhausted to the atmosphere. Theresulting polyethylene box had no discoloration, the impact strength was5000 ft. lbs/inch and warpage was equivalent to that -obtained inExample 1.

While the details of the process are given to a suflicient extent toenable the process to be used, variations and modifications may beresorted to without sacrificing the benefits of the invention.

The process has been described with particular reference to powderedlinear polyethylene of a density range of 0.945 to 0.970 gm./cc., and amelt index of 1.0 to 20.0 gm./l0 min. and the internal cooling of suchmaterials constitutes a preferred application. However, it is understoodthat the process is not limited to any particular plastic; for example,conventional polyethylene of low and medium density, ethylenecopolymers, polypropylenes, polyvinyl chlorides, etc., may also beemployed.

The process has been described with reference to a box mold, but it isunderstood that the process is not limited and can be used for anyhollow, open ended or divided object by suitable changes in the mold,gas entry and discharge lines, etc.

While the preceding description refers to the use of cooling head 32after the mold is removed from the oven, it is possible to installcooling head 32 at the beginning of the molding process. In this case, anon-adherent material with low thermal conductivity would be connectedto nozzle 53 of FIGURE 3 to prevent the polymer from clogging the ventsto enable an uninterrupted molding cycle. Moreover, it should beunderstood that the mold opening receiving the venting and cooling meanscould be much smaller than that shown in FIGURE 1.

It is also understood that the process lends itself to various othertypes of internal cooling medium, for example, atomized water, spray,steam, carbon dioxide, etc. The process is not restricted by the type ofexternal cooling nor by whether the mold is rotating during cooling.

Although this invention has been described with respect to its preferredembodiments, it should be understood that many variations andmodifications will now be obvious to those skilled in the art, and it ispreferred, therefore, that the scope of the invention be limited not bythe specitic disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:

1. A rotational molding apparatus for molding an open-end articlecomprising a mold, a lid for said mold, said lid being lined with aninsulating panel, said lid being provided with a centrally locatedopening, a cooling head receivable within said opening and dependingtherethrough, rotation mean connected to said mold for rotating saidmold about orthogonal axes, connection means for connecting said lid andcooling head to said mold for enclosing said mold, a said cooling headincluding channel means extending between the exterior of said mold andthe interior of said mold, and a source of cooling fluid connected tosaid channel for introducing cooling uid into the interior of said moldwhile said mold is rotating about its said orthogonal axes..

2. Rotational molding apparatus as set forth in claim 1 where ,saidcooling head includes a supporting plate through which said channelmeans extends, an upstanding rod mounted upon said plate for engagementwith said connection means for locking said cooling head and lid in moldclosing position, and a nozzle mounted upon the lower extremity of saidchannel means, said nozzle being provided with a plurality of radiallydisposed internal channels communicating with said channel means fordirecting fluid cooling medium against the interior wall surface of saidmold, said supporting plate being provided with a plurality of uniformlyspaced apertures for the venting of fluid cooling medium from said mold.

3. The device as set forth in claim 2 wherein said References CitedUNITED STATES PATENTS 2,926,387 3/ 1960 Lombardo.

2,979,778 4/ 1961 Simons 264-85 3,095,260 6/ 1963 Ferriot 264-3 11FOREIGN PATENTS 1,312,348 11/ 1962 France.

ROY B, MOFFITT, Primary Examiner,

1. A ROTATINAL MOLDING APPARATUS FOR MOLDING AN OPEN-END ARTICLECOMPRISING AMOLD, A LID FOR SAID MOLD, SAID LID BEING LINED WITH ANINSULATING PANEL, SAID LID BEING PROVIDED WITH A CENTRALLY LOCATEDOPENING, A COOLING HEAD RECEIVABLE WITHIN SAID OPENING, A COOLING HEADRECEIVABLE WITHIN SAID OPENING AND DEPENDING THERETHROUGH, ROTATION MEANCONNECTED TO SAID MOLD FOR ROTATING SAID MOLD ABOUT ORTHOGONAL AXES,CONNECTION MEANS FOR CONNECTING SAID LID AND COOLING HEAD TO SAID MOLDFOR ENCLOSING SAID MOLD, A SAID COOLING HEAD INCLUDING CHANNEL MEANSEXTENDING BETWEEN THE EXTERIOR OF SAID MOLD AND THE INTERIOR OF SAIDMOLD, AND A SOURCE OF COOLING FLUID CONNECTED TO SAID CHANNEL FORINTRODUCING COOLING FLUID INTO THE INTERIOR OF SAID MOLD WHILE SAID MOLDIS ROTATING AOUBT ITS SAID ORTHOGONAL AXES.